1. Field of Invention
This invention is related to liquid electrophoretic development of latent electrostatic images. More particularly, this invention is directed to systems and methods for measuring and for correcting for changes in the developability of liquid toner color components.
2. Description of Related Art
In liquid electrophoretic image forming systems, liquid developer materials include a liquid carrier material and charged toner particles dispersed into the liquid carrier material. The liquid developer material is applied to a photoreceptor or a dielectric member carrying a latent electrostatic image. The toner particles suspended in the liquid carrier material are attracted toward the image areas of the latent electrostatic image on the photoreceptor or the dielectric member to form a developed liquid image. The developed image on the photoreceptor or the dielectric member may be subsequently transferred either directly to a substrate or to an intermediate transfer member for later transfer to the substrate.
Liquid toner particles include colored material, such as pigments and/or dyes, and/or resin. An additive, called a charge director, may be dissolved in the carrier liquid to impart a charge to the toner particles. An additive, called a charge control agent, may be included in the toner particles to control the charge on the toner particles. Other additives may also be included in the liquid toner to improve its performance.
Liquid electrophoretic toners have been used in various ways in a number of conventional devices. The Xerox ColorgrafX 8954 and similar devices develop the liquid toner onto an electrostatic image written onto a dielectric paper. The Savin 870 printer develops a single color toner onto an electrostatic image on a photoconductor drum. The developed image is then transferred to a final substrate. The AM Graphic Electropress develops two differently-colored toner images onto two photoconductor drums and sequentially transfers the differently-colored toner images to a final substrate. The Indigo E-Print 1000 develops four process color (cyan, magenta, yellow, and black) images onto a photoconductor drum, transfers the four differently-colored toner images to an intermediate member, and then transfers the images from the intermediate member to the final substrate.
Liquid toners have many advantages over powder toners and often produce images of higher quality than images formed with powder toners. For example, images developed with liquid toner may adhere to the copy substrate without requiring fixing or fusing of the image to the copy substrate. In addition, the toner particles suspended in the liquid carrier material can be made significantly smaller than the toner particles used in powder toners. Using such small toner particles is particularly advantageous in multicolor processes where multiple layers of toner particles generate the final multicolor output image. Additionally, multicolor output images made with liquid toners generally have a significantly more uniform finish compared to images formed using powder toners.
Liquid toners typically contain about 0.5-10% by weight of fine solid particulate toner material dispersed in the liquid carrier material. The liquid carrier material is typically a hydrocarbon. After developing the latent electrostatic image, the developed image on the photoreceptor or the dielectric member may contain about 10-20% by weight of the solid particulate toner suspended in a residual layer of the liquid hydrocarbon carrier. To complete the development process, the excess liquid carrier material is typically removed from the photoreceptor or the dielectric member.
Mixing multiple color toners to produce a particular color developing material, in analogy to processes used to produce customer selectable color paints and inks, has generally been extended to electrostatographic printing technology, as disclosed, for example, in commonly assigned U.S. Pat. No. 5,557,393. This mixing includes forming an electrostatic latent image on an image forming device. The electrostatic latent image is then developed on the image forming device with at least one dry powder developer containing carrier particles and a blend of two of more compatible toner compositions. The developed toner image is then transferred to and fixed to a receiving substrate. Among the compatible toner compositions that may be selected are toner compositions having blend compatibility components coated on an external surface of the toner particles or particulate toner compositions containing blend compatibility components or passivated pigments. Electrostatographic imaging devices, including a tri-level imaging device and a hybrid scavengeless development imaging device, can also be used to create multi-color images using this process
U.S. Pat. No. 5,713,062 discloses systems and methods for controlling color mixing in an electrostatographic printing system. In the 062 patent, an operative mixture of a colored developing material is continuously replenished with selectively variable amounts of the developing materials of the basic color components that make up the operative mixture. The rate of replenishing the various color components added to the operative mixture is controlled to provide a mixture of developing material capable of producing a customer-selectable color on an output copy substrate. A colorimeter is provided to monitor the color of a test image printed with the operative mixture of the developing material, so that the color of the operative mixture can be brought into agreement with a color required to produce the customer-selectable output color.
U.S. Pat. No. 5,012,299 discloses a color adjustment apparatus for an electrostatographic printing machine. The color adjustment apparatus includes a color chart that visually represents all real colors in terms of color elements of saturation and hue, which can be selected using a touch key. The selected colors can be used to create highlight or spot colors on a printed image. The selected colors are obtained by combining halftones of different primary color separations on a photoreceptor or an intermediate drum. That is, to obtain selected colors by combining primary colored toners, the differently-colored toners are sequentially printed onto a surface, rather than being combined as materials before developing and printing as a solid layer. For the reasons described above, such process color approximations to a customer-selected color will show greater solid area color variations and greater line raggedness. Furthermore, some customer-selected colors can not be as precisely matched by overlapping halftones as by printing a solid area using a mixture of primary colors.
U.S. Pat. No. 5,543,896 discloses methods for measuring tone reproduction curves using a single structured patch. Development control is provided by storing a reference tone reproduction curve. A single test pattern, including a scale of pixel values, is formed in an interdocument zone on a photoreceptor surface. The test pattern is sensed in the interdocument zone. A control response to sensing the test pattern is provided relative to the tone reproduction curve to adjust the print quality.
U.S. Pat. No. 5,369,476 discloses toner control systems and methods for electrographic printing. In the 476 patent, toner is delivered from a reservoir to a toner fountain and applied to an electrostatically charged sheet to form an image. The visual quality of the image is monitored. Toner concentrate is added to the toner in response to the monitored quality to increase the amount of pigment particles in the toner, to maintain a substantially constant image quality. As disclosed in the 476 patent, a test image is formed outside the main image on the sheet. The brightness of one or more predetermined colors in the test image is monitored.
Conventionally, print engines using liquid electrophoretic toners attempt to keep constant the developability of the toner by measuring and keeping constant the volume of the toner, the concentration of the toner particles in the toner and/or the conductivity of the toner. The concentration of the toner particles in the toner is the mass of the toner particles divided by the total mass of the toner, including the mass of the carrier liquid. Methods for measuring these properties and adding one or more toner components in response to changes in measured property values are known as toner replenishment methods. Toner replenishment methods are described in "Control of Liquid Electrophoretic Toner Supplies," by Gibson et al., pp. 214-217, IS&T's NIP 14: International Conference on Digital Printing Technologies, Toronto, Canada, Oct. 18-23, 1998 (Gibson) which is incorporated herein by reference in its entirety.
U.S. patent application Ser. No. 08/721,419 discloses mixing liquid inks to match a customer-selected color, including measuring the transmission spectrum of the mixed ink, using the known absorption spectra of the primary inks which form the mixed ink, determining the concentrations of the primary inks in the mixed ink, and adding primary ink concentrates as needed to keep the primary ink concentrations at the target values appropriate to the customer-selected color.
U.S. patent application Ser. No. 09/093,703 discloses mixing liquid inks to match a customer-selected color, including measuring light transmitted through the mixed ink and each of several optical filters, use of a priori information about the primary inks, determining the primary ink concentrations, and adding primary ink concentrates as needed to keep the primary ink concentrations at the target values appropriate to the customer-selected color.
U.S. Pat. No. 5,240,806 discloses a liquid color toner composition usable with both contact electrostatic transfer processes and gap electrostatic transfer processes. The liquid toner comprises a colored predispersion including a non-polymeric resin material having certain insolubility (and non-swellability), melting point and acid number characteristics, an alkoxylated alcohol having certain insolubility (and non-swellability) and melting point characteristics, and a colorant material having certain particle size characteristics. The liquid color toner further comprises an aliphatic hydrocarbon liquid carrier having certain conductivity, dielectric constant, and flash point characteristics.
U.S. patent application Ser. No. 08/831,454 discloses controlling the composition of a mixed ink by measuring the color of a printed patch and by applying rules for adjusting primary color concentrations based on differences between the printed patch and a customer-selected color.
There are a number of known control systems for controlling electrostatographic processing parameters in response to the quality of the produced image using a test image or patch. For example, it is common to provide a scanning device to sense the optical density or other characteristics of a development test patch to generate a control signal to adjust the print quality.